Method of forming a fluorescent screen



May 31, 1938. J. c. BATcHELoR METHOD 0F FORMING A lFLUORESCENT SCREEN Filed Dec. 21, 1934 2 sheets-sheet 1 INVENToR.

May 431, "1938.

J. c. BATcHELoR 2,119,309

METHOD VOF FORMING vA FLUORESCENT SCREEN Filed Dec. 21, 1934 2 Sheets-Sheet 2 VIVE- Ilc- INVNTOR.

.aka/Ww#- Patented May 431, 1938 uNirED STATES PATENT OFFICE METHOD OF FORMING A FLUORESCENT l SCREEN My invention relates to improvements in methods of forming a screen on material, and, more particularly, a iiuorescent screen on a cathode ray tube.

My invention relates, more particularly, to improvements in the methodv of making the light producing structure of 'a cathode ray tube designed for the reproduction of television images and related arts.

The requirements of a fluorescent screen for television service are considerably more rigorous than those for a screen for oscillographic work. It is apparent that -if a screen were not homogeneous in thickness. throughout its area, a television image reproduced thereon would suffer considerable loss in denition in certain portions of its area owing to the varying degree of optical transmission eiciency and diffusion of light resulting from the variationsin thickness. Furthermore, it is' of great importance that the optical elciency be maintained very high in order that unreasonably highv beam accelerating potentials will not be required to reproduce images of Sufficient `brilliancy and contrast. This must be accomplished largely by adjusting the thickness of the screen' quite accurately.

Ira-addition, it is necessary, owing to the relative lack of skill of the user of such tubes, that the screen be extremely rugged and tenacious in order to prevent its being harmed by careless handling. For several reasons, this tenacity preferably should not be caused by the introduction of a'binding material. Inasmuch as most television tubes are of the high vacuum type in order to minimize the velocity modulation effects which gas would cause, and that vacuumI must be maintained extremely high in view of the relatively high accelerating potentials used, there would be required an extremely rigorous exhaust schedule 40\t0 outgas the binding material in order to prevent the evolution of gas subsequent to sealing the tube from the exhaust system, and the spoiling of the tube which would result from the presence of such gas. Furthermore, a binding material, even if it did not impair the vacuum in the tube, would necessarily envelope the particles of fluorescent material and absorb an appreciable part of the energy from the incident electrons before they should reach the iiuorescent material. 'This absorption of energy would materially reduce the ltranslation eilciency of the screen, and it is therefore undesirable.

Still further, in certain types of `cathode ray tubes to which my invention is particularly ap- 55 plicable, it is customary, in the absence of electrical conductivity of the fluorescent material, to rely upon the secondary emission characteristics of the uorescent material itself for dissipation of the electrical charge which would otherwise accumulate from the presence of the unneutralized incident electrons. It is possible to produce fluorescent materials synthetically which; in addition to beingv unusually eillcient electro-optically, give a quite large amount of secondary e'mission upon electronic bombardment. Obviously, if the particles of this material were to be coated with a less active binding material, the electrical charge would be dissipated less readily and the eillciency of the screen would be reduced.

Furthermore, in manufacturing cathode ray tubes for television use, the quantity requirement is such that the screen must be relatively easily applied and the time required for the application must be as short as is compatible with the neces- :sary high quality.

With all the foregoing in mind, it is an object of my invention to provide a high quality fluorescent screen relatively quickly and with comparatively little expenditure of effort.

In accordance with my invention, a screen is applied to a surface or structure by covering that @surface or structure with a suspension of fluorescent material in a suitable liquid and causing the. iluorescentmaterial to be deposited on said surface or structure under the influence of centrifugal force, following which the liquid is removed by decantation. l

Further, in accordance with my invention, a screen is formed or produced on a surface such as glass, mica or metal in such a manner that no binding'material is required to impart the required tenacity, and therefore extreme purity of the fluorescent material may be maintained.

In order to disclose by invention more fully, attention is directed to the accompanying' drawings'. Figure l1 is a View of a simple mechanism capable of producing a screen as described in the specication; Figure 2 is a sectional view of one type of bulb to which my invention may be applied; Figure 3 is an enlarged sectional `view of a portion of a fluorescent screen produced by my process; Figure 4 is an enlarged sectional view of a portion of a fluorescent screen made by certain modifications of my invention; Figure 5 is a Schematic diagram of a circuit in which my tube may be used.

In Figure 1, a motor i is provided with its shaft 2 extending vertically above it. Mounted on the vshaft 2 is a member 3, capable of being.. rotated in a horizontal plane by the vmotor l. Supported near the end of the member 3 by the trunnions 4 and capable of being removed through the slots 5 are the two carriers 6 which are so designed as to receive vand hold firmly the two bulbs 1.

In Figure 2, a glass bulb 1 has a lead-in wire 8 sealed in its side following which a metallic film Si is inserted terminating in one extreme in a smooth circle at the edge of the at window I0 and in the other extreme in a smooth circle in the neck Il.

In practising my invention, two bulbs are prey pared with lsuitable lead-in wires 8 and metallic films 9 and are tlien mounted in the carriers 6 and allowed to hang vertically downward.

Any suitable fluorescent material such as calcium tungstate, zinc sulphide, willemite or other material may be used. The material is prepared by grinding and screening lso that the particle sizes lie within the correct limits to give reasonably rapid deposition of the screen and yet suiiiciently small grain structure for the picture to be produced. For example, I have found it convenient, when using willemite, to prepare the material as follows: the willemite is first crushed and ground in a ball-mill until the average particle size is that which will pass through a 270 mesh screen, and the material is then thoroughly dried and sifted through such a screen. It should be understood that a 270 mesh screen is used merely as an example; other sizes-are often desirable and are determined by the required characteristicsof the fluorescent screen to be made and the constants of the apparatusused in making the screen. Following the screening, the material is shaken into a suspension in a quantity of distilled water of the order of 20 times the weight of the willemite to be treated, and the suspension is then allowed to settle Afor a period of time, such as 5 minutes. The remaining suspension is then poured from the container in which remains 4Vthe portion of the willemite which settled from the suspension during the time allowed. This material remaining in the container comprisesparticles of substantially uniform size and is therefore well suited to certain embodiments of my invention. The material is then thoroughly dried, following which it is sometimes desirable to wash the material in ether or other suitable solvent in order to remove any grease or other foreign matter which may have been introduced in the grinding process. Following. this washing, the material is again thoroughly dried, vwhereupon it is in suitable condition to be used in preparing fluorescent screens.

The exact quantity of fluorescent powder required to produce the desired screen is then thoroughlyA shaken into a suitable quantity of suspending liquid such as distilled water, and the suspension is poured into the bulbs 1. The quantity of liquid used should be sufficient to cover the bottoms of the bulbs to a depth of several' millimeters. I have found 8 millimeters to be a suitable depth of suspension in the case of bulbs having flat bottoms, but in the case of bulbs where, for reasons of strength, optical characterf istics or other reasons, it is necessary that the ,surface on' .which the screen is to be deposited be \curved, the factor which determines the depth of liquid to be used is the ratio of the smallest depth to the greatest depth over the area on which the screen is to be deposited. For example, if the surface on which the screen is to be deposited is a' millimeters, I have found it desirable to use a depth of suspension of at least 18 millimeters in order that the ratio of the least depth, which is 18 millimeters in this case, to the greatest depth, which is 20 millimeters, will be of, the order of and thus the thickness of the fluorescent screen which is deposited from the suspension will not vary more than 10% throughout the area of the screen.

'I'he motor I is immediately set in motion, and under the influence of the centrifugal force, the carriers and'bulbs 6 and 'l are caused to assume the upper positions l2 as indicated by the broken lines. Under the influence of the centrifugal force, the particles of fluorescent material are caused to travel through the suspending liquid to the surface of the window i0 of the bulb 1, and the force further causes the material to be rmly packed in a uniform film` on the surface.

After a reasonable time ranging from a few seconds to several minutes, depending upon the distance from the shaft 2 to the windows I0 when the carriers are in the upper positions i2, the speed of rotation of the motor, the size and density of the particles and the density and viscosity of the suspending liquid, the power is disconnected from the motor and it is allowed to come to rest. The bulbs are removed from the carriers immediately, and the liquid is then poured from the bulbs and they are set to dry in a vertical position with the neck il down.

I have found that with a distance of 6 inches from the shaft 2 to the windows i0 with the carriers in the upper positionsV I2, with-a speed of rotation of 1800 revolutions per minute, with hours of settling under the-influence of gravity.

Obviously, therefore, definite advantages are obtained by the method according to my invention, for example, using relatively low speeds of rotation at a short radius. 'Ihe advantages referred4 to have been obtained when the centrifugal force is of the order of four times the force of gravity,

and when this force is created by a speed of rotation of approximately revolutions per minute at a radius of six inches. p l

It is apparent, of course, that in order further to increase the speed of production of fluorescent screens by my improved method, it is possible to use a rotating device having provision for a large number of tube containers on a single rotator, so that many screens may be produced in a single rotating operation. I

'I'he amount of fluorescent material placed in a suspension for a tube may vary widely, and will be determined by the required thickness of materiall on the screen; that thickness is in turn largely determined by the voltage at which the vtube is to operate and the density of the fluorescent'material. These two factors largely determine the amount of penetration of the screen by the electrons and consequently the optimum thickness for greatest optical efficiency. I have found that screens made of willemite and designed to be operated with an electron velocity corresponding to 5000 volts are quite efficient when made with 1.5 t'o 2.5 milligrams of material per square centimeter.

When fluorescent screens are made by my process it is possible to adjust the thickness of fluorescent material with much greater accuracy 75 a1 rasee than has heretofore been possible. Even when particles of various sizes are used in the fiuorescent material, the complete deposition of all particles from the liquor may be assured by applying the centrifugal force for a somewhat longer period than is computed to be required completely to throw down particles of the size or sizes known to be used, `and for this reason a measured amount of fluorescent material may be placed in suspension and the deposition of the entire amount may be assured. i

The depth-to which a material is penetrated by electrons in motion follows quite closely the equation:

D=Ic2 p in which:

D=depth of penetration lc: constant of proportionality v=electron acceleration voltage p= density of-material being penetrated Screens having the necessary homogeneity could be made by allowing the material to settle under the influence of gravity, thenl very gently decanting the liquor. The preparation of such a screen, however, would occupy several hours and in addition it would be far from ideally suited to the desired purpose. Owing to the relative smallness of the force of gravity as compared with that of rotation, the thickness of the screen with a given weight of material would be quite great and as a result there would be excessive diffusion of light produced at the back of .the screen as it passed to the front of the "screen,

In addition, there would be an undue amount of space charge in the voids between particles, thus greatly reducing the effective velocity of electrons upon impact with the fluorescent material. In order to understand this aspect of my invention better it is convenient to consider the manner of operation of a cathode ray tube, to which my invention is particularly suited.' Figure 5 shows a typical circuit employing a cathode ray tube embodying my invention. In this circuit, an electron emitting cathode 26 is heated by the battery 2l, the current from which is controlled by the rheostat 20, the electrons emitted are accelerated by the anode 20 under the influence of the battery 25 and controlled by the control element 30 which is biased by the battery 3| and modulated by any suitablesignal impressed across the resistor 02. Electrons ejected from the anode 29 are deflected by magnetic iieldsvset up by the defiecting coils 33 and 00 carrying current from the deflection generators 35 and 30, whereby the modulated electron beam is caused to explore rhythmically the fluorescent screen 23' to effect the desired representation thereon..`

Referring to Figure 3, the window I has deposited upon its inner surface I II, particles of fluorescent material I3 in a random manner such that certain points I5 stand out prominently for unimpeded bombardment by an electron beam proceeding along the line of the arrows I0 whereas certain other portions Il of the screen are approached by electrons from the beam following the path indicated by the arrows I8 to strike the uorescent particles I9 by penetrating the chambers formed by the particles of fluorescent material. It is apparent that, in penetrating such chambers, certain space charges will be set up within the walls thereof, which charges will appreciably impede the entrance of additional electrons along the beam l0. Thus the effectiveness of the fluorescent particles I9 is substantially diminished when the chambers between the upper fluorescent particles such as 20 are of substantial depth as compared with the diameters of the particles of fiuorescent material.

Furthermore, cathode ray tubes are often'con. structed having an electrically non-conducting fluorescent screen which is insulated from all conducting portions of the tube, and moreover the tube is exhausted to a high degree of vacuum so that no ionization is possible to provide a return path for electrons incident .upon the fluorescent screen from the electron beam. In such tubes, the remaining expedient for the removal of electrons from the fiuorescent screen is utilizing secondary emission of electrons from the fluorescent particles and maintaining a sumcient potential difference between the fluorescent screen 23 and the metallic film 0 (see Figure 5) to draw the secondary electrons along such' paths as that indicated by the arrow 2l (Figure 3)A from the fluorescent screen to the metallic film 0. In this fcase the operation of the tube with respect to dissipation of electric charge from. the fluorescent screen is as follows: assuming the tube is operated with the metallic lm 0 maintained at a potential. of 10,000 volts positive with respect to 'the cathode 20 by' the batteries 20 and 25, and assuming that at the moment the voltages are applied toy the tube no Vpotential difference exists between the cathode 20 and the fluorescent screen t0, electrons are accelerated toward the fluorescent screen and are caused to strike the screen with some finite velocity. Upon incidence of one such electron upon the fluorescent screen, more than one secondary electron is emitted from the nuorescent material and such secondary electrons are immediately attracted to the metallic film 0 leaving an unbalanced positive charge upon the fluorescent screen. Electronic bombardment of the fluorescent screen continues and the positive charge of the screen increases until it approaches in potential the potential upon the metallic film 9, but as it does so the electric field which serves to draw the secondary electrons from the fluo'y rescent screen 23 to the metallic film 9 decreases and at some value of potential dierence between the screen and the film, usually :from 1000 to 2000 volts, the attractive force off the film upon the secondary electrons becomes insufficient to draw the secondary electrons from the iiuorescent screen, so that at this point an equilibrium condition is established between the fluorescent screen and the metallic film 9. Under this condition of l operation the electrons will approach the fluorescent screen along the paths I0 and I0 with a velocity corresponding to 10,000 volts,`but, if the potential difference between the fluorescent screen 23 and the film 0 is- 1000 volts, the electrons following the paths. I6 will be decelerated to a velocity corresponding to 9000 volts, with which velocity they strike the particles 20, but the electrons following the paths I0 will be decelerated lto a velocity corresponding to 9000 volts less the potential. gradient through the space charge barrier in the chambers between the particles 20, and will strike the fluorescent particle I9 with a velocity corresponding toa voltage somewhat less than 9000 volts. By meansv of my improved method of producing fluorescent screens I have been able to so pack the fiuorescent particles that the volume of voids between the particles -of fluorescent material is small, andthe volume of such voids may readily be made considerably less than the total volume of said material, 'I'hus it is possible with my improved iluorescent screen to realize an operating condition' wherein the average potential difference existing between the iluorescent screen and the final accelerating 'anode vis less than 10% of the potential difference between the cathode material capable of passing through a 400 mesh screen so that the voids existing between vthe particles 20 will be illled by the particles 22 of smaller diameter, which particles are thrown out of suspension more slowly than are particles of larger diameter. It is apparent that in some cases it may be desirable to use still more sizes of particles for the purpose of minimizing the amount of voids existing between the fluorescent y particles.

The ratio of the forces exerted in packing the material centrifugally and by gravity is given by the equation:

1;: 4'rrn2 F in which:

Fc=centrifugal force exerted Fg=gravitational force exerted r'=radius of rotation n=speed of rotation :acceleration by gravity Fro this equation it can be shown-that with a radius of 6 inches and rotation of 3600 revolutions per minute the centrifugal force is more than 2000 times as great as the force caused by gravity, with the result that the thicknessof the screen is decreased and the tenacity is increased, and, at the Sametime, the time required for preparation of the' screen is very greatly reduced.

It is apparent that my invention may well be applied to the preparation of fluorescent screens other than those which are deposited directly upon a wall of a cathode ray tube. It is often desirable that a screen be prepared on a plate of mica, metal or the like subsequently to be inserted in a cathode ray tube. Such a screen may, obviously, be made by placing such a plate in the bottom of a cup, covering the plate with a suspension of fluorescent material as described and proceeding in a manner as'described for a cathode ray tube bulb.

It should be understood that my invention is in no way limited by the embodiments specifications may be devised by those skilled in the art without departing from the spirit of my invention or the scope of the claims.

I claim: v 1. In the art of making a fluorescent screen, the method which comprises applying to a supporting surface a fluorescent material in the form of relatively small particlessuspended in a liquid, applying centrifugal force to said particles to cause said particles to adhere to said surface in a firmly compacted illm, and subsequently removing the liquid.

2. In the art of making a fluorescent screen, the method which comprises applying to a supporting surface a iluorescent-material in the form of relatively small particles suspended in a liql uid, applying centrifugal force of at least the order of four times the force of gravity to said particles to cause said particles to adhere to said surface in `a rmly compacted film, and subsequently removing the liquid.

3. In the art of making a fluorescent screen, the method which comprises applyingto a supporting surface a fluorescent material in the form of particles of a size which will pass through a 200 mesh screen, said particles being suspended A in a liquid, applying centrifugal force of at least that produced on said particles rotating at revolutions per minute at al radius of six inches to cause said particles to adhere to said surface in a firmly compacted film, and subsequently removing the liquid.

.cally described, and it is apparent .that modifi- 

